Devices of this type serve the function of stabilizing the drill tool or the drill string in the drilled hole and to counteract deviations of the drill bit from the preset drilling direction. Undesirable deviations occur particularly in the transition zones between harder and softer layers in deep-hole drilling, especially if the drill crosses these layers at an acute angle.
Up to now it has been known to achieve stabilization of the drill tool through the incorporation of steering devices, so-called course indicator rods or stabilizers. These are rod-type pipes mounted with steering shoes, the outer diameter of which corresponds to the drilling diameter, and follow the drill bit, steering it in a concentric manner. Such devices are available in various designs, for example rigid designs, where the stabilizing shoes are rigidly connected to the multiple drill rod unit and rotate in the drill hole, or those of the type first mentioned, where the stabilizing shoes are provided on a separate outer pipe in which the multiple drill rod unit is positioned for rotation. Target drill rods are already available for vertical drilling, with built-in automatic vertical steering utilizing gravitational force and using the pressure of the flushing fluid or the rotation of the drill rods to develop steering pressure
Known rigid steering devices have the disadvantage that they counteract the deviation of the drill tool from the drilling direction only to an insufficient extent. A reason for this is the fact that the diameter of the drilling tool must always be dimensioned a little larger than the diameter of the steering device in order to avoid jamming of the steering device in the drill hole. This is important above all because the diameter of the drill hole may diminish because of the wear of the drill bit. Therefore, known steering devices are not always urged tightly against the drill hole wall during the drilling process, so that the drilling tool may wander off course.